[onboarding] additional fields in registration form

[dezhizhang1985]

Missing fields of the entity registration form:

• [x] ~IPA code or VAT number, that determines whether the participant is public or private~
• [x] ~at least one administrative email contact~
• [x] SPID/CIE authentication request trigger url at the onboarding system testing OP

[peppelinux]

@dezhizhang1985 we won't implement the first two but the url that trigger the auth (not required, it can be null).

I've included this issue in the milestone v0.6.0

[dezhizhang1985]

done here: https://github.com/italia/spid-cie-oidc-django/pull/189